Engineering light-matter interaction using two dimensional materials

Abstract

The recent discovery of a new class of two dimensional(2D) atomic crystals allows the possibility of strong coupling of electromagnetic waves with various collective excitations such as plasmons and phonons, and carries great potential for nanophotonics across the long sought after terahertz to mid-infrared spectrum. In this thesis, I will show a few examples of how light-matter interaction can be engineered in 2D materials, through the modification of both microscopic as well as macroscopic properties of such materials. I describe how the plasmons in graphene are modified by coupling with the optical phonons of the naturally hyperbolic material, hexagonal boron nitride (hBN). I examine theoretically the mid-infrared emission properties of graphene-hBN heterostructures derived from their coupled plasmon-phonon modes, leading to the appearance of tunable dips in the spontaneous emission spectra. Going beyond graphene, I consider a generic gapped Dirac system. I show that the valley imbalance due to pumping with a specific circular polarization, leads to a net Berry curvature, giving rise to a finite transverse conductivity. Using this model, I predict the appearance of nonreciprocal chiral edge modes, their hybridization and waveguiding in a nanoribbon geometry, and giant polarization rotation in nanoribbon arrays. Among macroscopic structural effects, I consider localized plasmon resonances in nanostructures of 2D materials and the development of transformation optics methods. I formulate a general semi-analytical framework for a system of discs, whereby emission and absorption properties of dark and bright plasmonic modes are studied, as a function of graphene doping. Furthermore, I employ an open quantum systems formalism to show that under certain conditions, both the dark and bright dipolar modes in this system can support vacuum Rabi splittings for the plasmon-emitter coupling. Secondly, I expand the concept of transformation optics by formulating a novel scheme that can tackle arbitrary spatial variations of 2D materials, which are usually described by a surface conductivity. The novel phenomena enabled by photonic modes in two dimensional materials together with the ideas proposed in this thesis provide a basis for engineering light-matter interaction and controlling energy flow at the nanoscale.